The multi-layered aquifer systems of the Po Valley are one of the largest freshwater reserves in Europe. This aquifer plays a key role in supplying water in a densely populated area that suffer the intensive metropolisation processes. These are responsible for the enormous increase of water demand, of the hydrogeological risk and of the climate change which renders this aquifer more vulnerable. These challenges can be tackled thanks to an accurate reconstruction of the subsurface geological model of plain lowland areas such as the Padana Plain, North Italy, requires the employment of up-to-date non-invasive geophysical methods specifically designed to maximise the quantity of additional subsurface information to aid in the reconstruction of conceptual hydrogeologic and geochemical models necessary for further construction of hydrodynamic ones.
The geoelectrical method, one of the oldest active, since 1926, and are most widely used geophysical techniques for the exploration of the subsurface at different depths: from few cm’s to thousands of meters depending, mainly among others, on the characteristics of the target especially those related to the presence of a measurable contrast of the physical property between target and surrounding materials.
The Electrical Resistivity Tomography (ERT) and the less commonly used Electrical Induced Polarization (EIP) techniques shall be discussed through the presentation of some 2D images obtained at a test site the framework of the work package, WARBO-Life+ EU project (concluded in 2015, http://www.warbo-life.eu/)), related to the artificial recharge activities of an high salinity shallow subsurface aquifers. Such images, as shall we see later, allow for better visualization of lateral as well as vertical resistivity variations that can be interpreted in terms of lithology, saturation, physio-chemical modifications due to interaction with the natural and anthropogenic processes (Archie 1942; Mualem and Friedman, 1991; Ewing and Hunt 2006). All these factors represent in one way or another the ingredients that contribute in the reconstruction of a hydrogeological model especially where there is a need to characterize low land areas such as the Po Plain in northern Italy. In this area lateral and vertical lithologic variations are rarely uncommon. The sensibility of resistivity to groundwater saturation and quality, allowed their use since the sixties of the last century to map freshwater-saltwater interface mainly along coast lines. Further hardware and software advancements permitted the extension of their use to achieve suggestive 2D, 3D and recently 4D images of the resistivity and, less common, chargeability distribution in the subsurface. The 4D imaging, has recently been utilized to capture temporal resistivity variations of the subsurface resistivity provoked either naturally or artificially. As an example we present the application of static and dynamic ERT surveys carried out in a test site located at Copparo town, NE Italy (Fig. 1), Artificial Recharge (A.R.) operations were experimented to counteract the effect of undergoing salinization process of the shallow and the first semi-confined aquifer. It worth’s mentioning that this site is located at more than 60 km to the west of the Adriatic Sea coast. The high salinity is due to the plume of methane fossil waters that rise to the surface along tectonic discontinuities, stratigraphic gaps and mining wells abandoned in 1962 since it was forbidden the methane extraction to prevent subsidence induced by the overexploitation and to mitigate the salinization of channels for the irrigation and of green farmland induced by the dispersion of fossil waters that are very rich in chlorides, bromides and metals toxic to the environment and health (Sciarra et al 2015). The geochemical characterization of confined aquifers of the Po Valley in the northern sector of the Ferrara territory have identified in the deep water of fossil alluvial aquifers the contamination source that causes widespread salinization (Sciarra, 2015, Bonzi et al 2016). The Dominant ions of deep wells are rich in sodium chlorides and bicarbonates of magnesium and calcium (Mg/Ca ratio is about 1.5) and significant are the amounts of Potassium. Very low is the concentration of sulphates.
In fact, a new discipline of research projects has been developed in this direction in the last few years whose aim is to Manage Artificial Recharge activities (MAR projects, (Nieto et al 2012, Pezzi et al., 2014). This means that their use do not focus only on the geometrical definition of aquifer boundaries but goes a step further towards monitoring the dynamic behaviour of the salinization/de-salinization process in strict relation to the modality being used for A.R. Within this context, the main focus of the present work is to present and discuss the use of these techniques as a cost-effective tool for the monitoring of A.R. activities which constitutes one of the fundamental pillars of any MAR project.